Optical encoder systems are typically employed as motion detectors in applications such as closed-loop feedback control in motor control systems. By way of example, many optical encoder systems are configured to translate rotary motion or linear motion into a two-channel digital output for position encoding.
Many optical encoder systems employ an LED as a light source. In transmissive encoder systems, the light is collimated into a parallel beam by means of a lens located over the LED. Opposite the emitter is a light detector that typically consists of photo-diode arrays and a signal processor. When a code scale such as a code wheel or code strip moves between the light emitter and light detector, the light beam is interrupted by a pattern of bars and spaces disposed on the code scale. Similarly, in reflective or imaging encoder systems, the lens over an LED focuses light onto the code scale. Light is either reflected or not reflected back to the lens disposed over the photodetector. As the code scale moves, an alternating pattern of light and dark patterns corresponding to the bars and spaces falls upon the photodiodes. The photodiodes detect these patterns and corresponding outputs are processed by the signal processor to produce digital waveforms. Such encoder outputs are used to provide information about position, velocity and acceleration of a motor, by way of example.
In some three channel optical encoder systems, an index channel light detector is provided comprising photodiode arrays I and I/. Such encoders require increased surface area and package size to implement, however, and also suffer from an increased probability of misalignment between the code scale and the light detectors, seeing as light detectors are required over an increased surface area. Manufacturing costs and times are also typically increased in such an approach because specialized optical alignment equipment and steps are required. See, for example, U.S. Pat. No. 4,451,731 to Leonard.
In other three-channel optical encoder systems, an additional track is provided by a reference mark imprinted on a code strip or code wheel and photointerrupters disposed well to either end of an encoder. The reference mark is typically placed near the end of a code strip or code wheel, and is used to provide an indication that a complete linear motion (or motion limit) of a motor or other device connected to the encoder has been completed or achieved. When the motor or other device reaches its end-of-motion or other limit, a digital signal is generated by the encoder, which is employed in a feedback loop to stop the motor or other device from moving further.
Such an encoder system 10 is illustrated in FIG. 1, where encoder 8 is disposed on lower substrate 9 and below code scale 15, which is configured to move laterally above encoder 10. Photointerrupters/photodiodes 5 and 7 are disposed well from both ends of encoder 8, and are configured to detect an end-of-motion or limit signal generated by either end of code scale 15 being located directly thereabove. As will be appreciated, the prior art system of FIG. 1 is relatively large owing to photointerrupters/photodiodes 5 and 7 being located a fair distance from both ends of encoder 8. Moreover, the use of photointerrupters/photodiodes 5 and 7 requires that components in addition to encoder 8 be used, and that electrical connections be established to such components using, for example, extra lengths of flex cable. These factors result in an optical encoder system that is relatively large, requires additional steps in the manufacturing process to assemble the photointerrupter and related components, and also lower production output and increase cost.
A three-channel optical reflective encoder system is disclosed in U.S. Pat. No. 7,394,061 to Bin Saidan et al. (hereafter “the '061 patent”). In the '061 patent, a third index channel output signal is provided through the relatively complicated processing of signals corresponding to the A, A/, B and B/ data channels. In such an encoder, considerable resources and time must be devoted to designing the complex output circuitry required to produce the index channel output signal. In addition, the outputs provided by the data and index channels may be degraded if the total currents generated by light detectors are insufficient to produce the required output signals. As a result, at least certain pairs of A, A/, B and B/ channels cannot be employed to produce the index channel output signal. At higher resolutions, the two-channel design of the '061 patent fails, as the widths and corresponding surface areas of the data channel light detectors are small and the current they generate is insufficient to generate an index output pulse. Additional electronic circuitry is therefore required to increase photodiode current.
Referring now to FIG. 2, there is shown a conventional two-channel optical encoder 8 comprising integrated circuit 16 having single track light detector 18 forming a portion thereof, and mounted on encoder substrate 12. Single track light detector 18 comprises A, A/, B and B/ data channel photodiodes, as is known in the art. Light emitter 20 emits light upwardly towards a code scale (not shown in FIG. 1), where light is reflected downwardly towards single track light detector 18 through single dome lens 14. FIG. 3 shows conceptually how the size of integrated circuit 16 is increased when additional conventional indexing circuitry and corresponding indexing photodiode(s) are added to single track light detector 18 to form a three-channel optical encoder 8. In FIG. 3, border 6 represents the increased surface area consumed by three-channel integrated circuit 16 of FIG. 3 with respect to two-channel integrated circuit 16 of FIG. 2. The additional indexing circuitry and photodiodes of integrated circuit 16 in FIG. 3 prevent the resulting optical encoder package from being made small, and also result in considerable space in integrated circuit 16 being unused and therefore wasted.
The market demands ever smaller and higher resolution optical reflective encoder systems that can be provided without the use of complicated, expensive, signal processing output circuitry and without the use of photointerrupters mounted on large substrates that inhibit or prevent system miniaturization.